A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as laser diodes, are typically constructed from materials such as gallium arsenide and aluminum gallium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example, can be in excess of 40 percent.
The use of flashlamps, light-emitting diodes (as used herein, this term includes superluminescent diodes and superluminescent diode arrays) and laser diodes (as used herein, this term includes laser diode arrays) to optically pump or excite a solid lasant material is wellknown. Lasant materials commonly used in such solid state lasers include crystalline or glassy host materials into which an active material, such as trivalent neodymium ions, is incorporated. Detailed summaries of conventional crystalline lasant materials are set forth in the CRC Handbook of Laser Science and Technology, Vol. I, M. J. Weber, Ed., CRC Press, Inc., Boca Raton, Fla., 1982, pp. 72-135 and in Laser Crystals, Vol. 14 of the Springer Series in Optical Sciences, D. L. MacAdam, Ed., Springer-Verlag, New York, N.Y., 1981. Conventional host materials for neodymium ions include glass, yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12, referred to as YAG), YAlO.sub.3 (referred to as YALO), LiYF.sub.4 (referred to as YLF), and gadolinium scandium gallium garnet (Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12) referred to as GSGG. By way of example, when neodymium-doped YAG is employed as the lasant material in an optically pumped solid state laser, it can be pumped by absorption of light having a wavelength of about 808 nm and can emit light having a wavelength of 1064 nm.
U.S. Pat. No. 3,624,545 issued to Ross on Nov. 30, 1971, describes an optically pumped solid state laser composed of a YAG rod which is side-pumped by at least one semiconductor laser diode. Similarly, U.S. Pat. No. 3,753,145 issued to Chesler on Aug. 14, 1973, discloses the use of one or more light-emitting semiconductor diodes to end pump a neodymium-doped YAG rod. The use of an array of pulsed laser diodes to end pump a solid lasant material such as neodymium-doped YAG is described in U.S. Pat. No. 3,982,201 issued to Rosenkrantz et al. on Sept. 21, 1976. Finally, D. L. Sipes, Appl. Phys. Lett., Vol. 47, No. 2, 1985, pp. 74-75, has reported that the use of a tightly focused semiconductor laser diode array to end pump a neodymium-doped YAG results in a high efficiency conversion of pumping radiation having a wavelength of 810 nm to output radiation having a wavelength of 1064 nm.
Materials having nonlinear optical properties are well-known. For example, U.S. Pat. No. 3,949,323 issued to Bierlein et al. on Apr. 6, 1976, discloses that nonlinear optical properties are possessed by materials having the formula MTiO(XO.sub.4) where M is at least one of K, Rb, Tl and NH.sub.4 ; and X is at least one of P or As, except when NH.sub.4 is present, then X is only P. This generic formula includes potassium titanyl phosphate, KTiOPO.sub.4, a particularly useful nonlinear material. Other known nonlinear optical materials include, but are not limited to, KH.sub.2 PO.sub.4, LiNbO.sub.3, KNbO.sub.3, .beta.-BaB.sub.2 O.sub.4, Ba.sub.2 NaNb.sub.5 O.sub.15, LiIO.sub.3, HIO.sub.3, KB.sub.5 O.sub.8 .multidot.4H.sub.2 O, potassium lithium niobate and urea. A review of the nonlinear optical properties of a number of different uniaxial crystals has been published in Sov. J. Quantum Electron., Vol. 7, No. 1, Jan. 1977, pp. 1-13. Nonlinear optical materials have also been reviewed by S. Singh in the CRC Handbook of Laser Science and Technology, Vol. III, M. J. Weber, Ed., CRC Press, Inc., Boca Raton, Fla., 1986, pp. 3-228.
The conversion of optical radiation of one frequency to optical radiation of another frequency through interaction with a nonlinear optical material is well-known and has been extensively studied. Examples of such conversion include harmonic generation, optical mixing and parametric oscillation. Second-harmonic generation or "frequency doubling" is perhaps the most common and important example of nonlinear optics wherein part of the energy of an optical wave of angular frequency .omega. propagating through a nonlinear optical crystal is converted to energy of a wave of angular frequency 2.omega.. Second-harmonic generation has been reviewed by A. Yariv in Quantum Electronics, Second Ed., John Wiley & Sons, N.Y., 1975 at pages 407-434 and by W. Koechner in Solid State Laser Engineering, Springer-Verlag, N.Y., 1976 at pages 491-524.
Electromagnetic waves having a frequency in the optical range and propagating through a nonlinear crystal induce polarization waves which have frequencies equal to the sum and difference of those of the exciting waves. Such a polarization wave can transfer energy to an electromagnetic wave of the same frequency. The efficiency of energy transfer from a polarization wave to the corresponding electromagnetic wave is a function of: (a) the magnitude of the second order polarizability tensor, since this tensor element determines the amplitude of the polarization wave, and (b) the distance over which the polarization wave and the radiated electromagnetic wave can remain sufficiently in phase.
The coherence length, l.sub.c, is a measure of the phase relationship between the polarization wave and the radiated wave which is given by the following relationship: EQU l.sub.c =.pi./.DELTA.k
where .DELTA.k is the difference between the wave vectors of the polarization and electromagnetic waves. More specifically, the coherence length is the distance from the entrance surface of the nonlinear optical crystal to the point at which the power of the radiated electromagnetic wave will be at its maximum value. Phase-matching occurs when .DELTA.k=0. The condition .DELTA.k=0 can also be expressed as n.sub.3 .omega..sub.3 =n.sub.1 .andgate..sub.1 .+-.n.sub.2 .omega..sub.2 where .omega..sub.3 =.omega..sub.1 .omega..sub.2 ; .omega..sub.1 and .omega..sub.2 are the frequencies of the input electromagnetic waves; .omega..sub.3 is the frequency of the radiated electromagnetic wave; and n.sub.1, n.sub.2 and n.sub.3 are the refractive indicies of the respective waves in the nonlinear optical crystal. In the special case of second harmonic generation, there is incident radiation of only one frequency, .omega., so that .omega..sub.1 =.omega..sub.2 =.omega. and .omega..sub.3 =2.omega..
For appreciable conversion of optical radiation of one frequency to optical radiation of another frequency in a nonlinear optical crystal, the interacting waves must stay substantially in phase throughout the crystal so that: EQU .vertline..DELTA.k.vertline.=.vertline.k.sub.3 -k.sub.1 -k.sub.2 .vertline.&lt;2.zeta./1
where k.sub.1, k.sub.2 and k.sub.3 represent the wave numbers corresponding to radiation of frequencies .omega..sub.1, .omega..sub.2 and .omega..sub.3, respectively, and 1 is the interaction length in the nonlinear material. The term "substantially phase-matched," as used herein, means that .vertline..DELTA.k.vertline.&lt;2.pi./1 for a given nonlinear optical crystal.
A conventional method for achieving phase-matching in a nonlinear optical material utilizes the fact that dispersion (the change of refractive index with frequency) can be offset by using the natural birefringence of uniaxial or biaxial crystals. Such crystals have two refractive indicies for a given direction of propagation which correspond to the two allowed orthogonally polarized propagation modes. Accordingly, by an appropriate choice of polarization and direction of propagation, it is often possible to achieve phase-matching in a birefringent non-linear optical crystal. The term "phase-match axis," as used herein, refers to a line or direction through a non-linear optical crystal along which the substantially phase-matched conversion of a stated input radiation into a stated output radiation is permitted for at least certain polarizations of said input radiation.
Phase-matching is generally of either Type I or Type II. Type I phase-matching requires that the incident waves interacting in the nonlinear optical material have the same polarization. Type II phase-matching requires that the incident waves interacting in the nonlinear optical material have orthogonal polarizations.
Second harmonic generation within the cavity of a multilongitudinal mode laser by an intracavity doubling crystal has recently been analyzed by T. Baer, J. Opt. Soc. Am. B, Vol. 3, No. 9, pp. 1175-1180 (1986). This report sets forth an experimental and theoretical evaluation of the output of a Nd:YAG laser which is pumped by a laser diode array and contains an intracavity doubling crystal. It is reported that large amplitude fluctuations and longitudinal mode instabilities result when the doubling crystal is inserted into the laser cavity. However, it is also reported that these instabilities disappear when the laser is restricted to a single oscillating mode by an intracavity etalon. A detailed theoretical analysis of a multilongitudinal mode intracavity-doubled laser has been reported by X. G. Wu et al., J. Opt. Soc. Am. B, Vol. 4, No. 11, pp. 1870-1877 (1987).
U.S. Pat. Nos. 4,656,635 (Apr. 7, 1987) and 4,701,929 (Oct. 20, 1987), both issued to Baer et al., disclose a laser diode pumped, intracavity frequency-doubled, solid state laser. In these patents, it is stated that a problem with such devices is the generation of amplitude noise, including large amplitude spikes, which prevent or limit use in applications requiring a highly stable or constant output. It is further stated that this noise results from the combination of multiple longitudinal modes. However, it is disclosed that such noise can be reduced or eliminated by inserting an etalon into the laser cavity and thereby forcing the laser to operate in a single mode. It is also disclosed that it may be possible to reduce this noise by mode locking the laser.
U.S. Pat. No. 3,619,637, issued to Goto et al. on Nov. 9, 1971, discloses a harmonic generator which comprises at least two nonlinear dielectric crystals where: (a) the input and output surfaces of the crystals are parallel to each other; (b) the crystals are phase-matched for the conversion of input radiation to a harmonic; (c) input radiation is directed into the crystals perpendicular to the input surfaces; (d) the crystals are disposed in cascade; and (e) the optical axes of adjacent crystals, as projected on their input surfaces, intersect each other at right angles. It is further disclosed that such a device results in the efficient generation of harmonic radiation irrespective of the orientation of the plane of polarization of the input radiation. Similarly, U.S. Pat. No. 4,510,402, issued to Summers et al. on Apr. 9, 1985, is directed to an optical harmonic generator which contains a serially arranged pair of uniaxial birefringent crystals which can be positioned for either second or third harmonic generation. However, neither of these patents contains any suggestion that multiple nonlinear optical crystals could be inserted within the optical cavity of a laser.